Generally, construction machines such as hydraulic excavators have a crawler unit (undercarriage) having a crawler frame as a main body; an upper structure mounted on the crawler unit so as to turn around freely upon the crawler unit; and a work implement, cab, engine and others which are mounted on the upper structure.
The crawler frame includes a center frame for supporting the upper structure so as to turnable upon the center of the crawler frame and track frames coupled to the right and left sides of the center frame, extending in a longitudinal direction. The right and left track frames each support an idler and a drive wheel at the front and rear ends thereof respectively. The center frame is composed of a central frame section for supporting a swing bearing and legs which are disposed on the right and left sides of the central frame section for coupling the central frame section to each track frame. The whole center frame is made from sheet metal and is substantially H-shaped or X-shaped in its plan view. A known structure for such a center frame is such that in order to sustain the load imposed on the swing bearing positioned at the center, a desired number of vertical wall members are joined by welding between an upper face plate on which the swing bearing sits and a lower face plate located under the upper face plate (See Japanese Patent Kokai Publications No. 8-72615, No. 11-93209 and No. 2000-230252).
This prior art center frame structure has however revealed the problem that since the right and left track frames are coupled to the central frame section by the legs which are formed by sheet metal work with use of steel plates, steel plates complicated in shape are involved and the number of parts is increased. As a result, welded places and therefore the number of welding processes increase, requiring tremendous manufacturing time and manufacturing cost.
In addition, since the upper faces of the legs made of steel plates are flat, the mud which has penetrated into the machine during operation and traveling adheres to and deposits on the upper faces of the legs. This mud penetrates into the swing bearing causing damage to it, or moves onto the top faces of the track frames, interfering with the rotation of the track carrier rollers and causing lopsided wear of the track carrier rollers.
The accumulated mud is removed by washing which, however, needs a lot of water for removal and many washing processes, resulting in increased cleaning cost. In addition, a large amount of mud is left in the washing site after washing a construction machine so that the liveries who rent construction machines such as hydraulic excavators are bothered by the problem of mud disposal.
As an attempt to solve the above problem, there has been proposed a center frame structure in which the legs are made of cast steel and the base section of each leg is provided with a flange which is welded to a side face of a box-shaped central frame section. This arrangement raises the possibility that not only the number of parts but also the number of welding processes can be reduced, leading to a considerable reduction in the number of processes and processing time. Further, by forming each leg into a tubular shape and making the upper face of the leg convex in cross-section, the problem of the adhesion/deposition of mud can be solved.
In this center frame structure including the cast legs, however, the box-shaped central frame section is also formed from a sheet metal and the legs are welded to the vertical walls made of a sheet metal. For this reason, this center frame leaves much to be desired in the structure of the vertical walls that serve as reinforcement members for sustaining the load imposed on the swing bearing as well as in the number of parts, and therefore further simplification of the structure is required.
When producing cast legs for use in a middle-sized or larger construction machine, a large-sized molding box for steel casting becomes necessary as such legs are large in outside dimension. In casting by use of a large-sized molding box, a flow of hot water and gas venting are slow and a complicated casting method is involved, so that casting defects are likely to occur and manufacturing cost increases.
The present invention is directed to overcoming the foregoing shortcomings and a first object of the invention is therefore to provide a crawler frame for a construction machine the structure of which is simplified by reducing the number of parts with legs of the center frame being formed from cast steel. A second object of the invention is to provide a crawler frame for a construction machine which has the merit of causing no problems in casting in addition to the merit of the first object.